1. Field of the Invention
The present invention relates to a defective-pixel-correction technology in which image signals associated with defective pixels that have occurred in a solid-state image-pickup device are corrected.
2. Description of the Related Art
In the related art, it has been known that, in a solid-state image-pickup device, such as a charge-coupled device (CCD) sensor and a complementary metal-oxide-semiconductor (CMOS) sensor, defective pixels, which output abnormal image signals, occur in the manufacturing process of the solid-state image-pickup device. It is very difficult to completely avoid the occurrence of the defective pixels in the manufacturing process of the solid-state image-pickup device. Accordingly, in the related art, a defective-pixel-correction technology has been realized, in which image signals output from defective pixels are corrected using image processing.
In a defective-pixel-correction method used in the related art, generally, signals output from defective pixels are corrected using interpolation processing in which signals output from normal pixels that are adjacent to the defective pixels and that have the same color as the defective pixels are used. In this case, when the solid-state image-pickup device is shipped from a factory, by evaluating a dark output and an exposure output under predetermined conditions, pixels having output values that fall outside a predetermined output range are determined as defective pixels. Then, data on the type of the defective pixels, such as black pixels and white pixels, address data on the defective pixels (coordinates X in the horizontal direction and Y in the vertical direction of image data), and data on output levels of the defective pixels are acquired as defect-correction data.
The defect-correction data is stored in a read-only memory (ROM) or the like, and the ROM is mounted in an image processing apparatus, such as an electronic camera. The image processing apparatus performs a defect-correction process using the defect-correction data.
Recently, because the number of pixels in the solid-state image-pickup device has increased, the solid-state image-pickup device has been capable of shooting a very fine still image. Furthermore, an electronic camera capable of displaying/recording not only a still image but also a moving image has been realized. When a moving image is displayed/recorded, in order to obtain a finer resolving power in the time direction, it is necessary to ensure a frame rate of at least approximately from 24 to 30 frames per second.
Additionally, in order to perform both a generation process, in which image data read from a large number of pixels is generated in a case in which a still image is shot, and a reading process, in which image data are read at a high frame rate in a case in which a moving image is shot, when a moving image is shot, the solid-state image-pickup device is designed so that the number of pixels from which image data output can be decreased. For example, when image data is read from all of the pixel regions of the solid-state image-pickup device at a high speed, a reading process in which image data is read from one pixel from among every predetermined number of pixels in each of the horizontal and vertical directions of the solid-state image-pickup device, i.e., a thinned-out reading process, is performed. As a result, the number of pixels from which image data is output can be decreased.
As another method for decreasing the number of pixels from which image data is output, a thinned-out pixel-addition reading process using a so-called pixel-combination unit is known. When the thinned-out reading process is simply performed, the sampling frequency is spatially reduced, resulting in an increase in noise. For this reason, in the thinned-out pixel-addition reading process, in the solid-state image-pickup device, pixel outputs to be not read are added to pixel outputs to be read to obtain added pixel outputs, and the added pixel outputs are read. As the thinned-out pixel-addition reading process, for example, a method disclosed in Japan Patent Laid-Open No. 2002-135793 has been suggested.
In addition, in particular, in a CMOS-type solid-state image-pickup device, a partially cut out reading process can also be easily performed, in which pixel data is read only from pixels disposed in a certain region of the solid-state image-pickup device.
Regarding a method for correcting defective pixels of the solid-state image-pickup device, when a reading method in which image data is read from the solid-state image-pickup device is changed, the address data on defective pixels, which is used as the above-described defect-correction data, needs to be also changed because the size of image data to be obtained is changed in accordance with the reading method.
Accordingly, suppose that, also when a moving image is read, defective pixels are to be corrected in the same manner. As the address data on the defective pixels, it is necessary to convert from address data corresponding to an image size for an all-pixel reading mode to address data corresponding to an image size for a case in which a moving image is read. Additionally, when the solid-state image-pickup device is shipped from a factory thereof, it is necessary to acquire several types of defect-correction data in advance for a case in which a moving image is read, and to store all of the data in a camera. In accordance with a reading method of the solid-state image-pickup device, it is necessary to switch defect-correction data that is to be applied among the several types of defect-correction data. Alternatively, it is necessary to extract defect-correction data for a case in which a moving image is read when a camera is assembled, and to write the defect-correction data into the camera.
Furthermore, there are the following problems. For example, in a case in which an address-conversion method is switched in accordance with a reading method of the solid-state image-pickup device, the size of memory used in a camera system is equal to that of the ROM used in a case in which a still image is shot. However, address re-conversion is performed for each reading method using firmware of the camera, the load of a conversion process is heavy. Accordingly, because plenty of time is necessary to perform the conversion process, it is very difficult to finish the conversion process in a realistically short time.
In particular, in defect-correction data that can be used in an all-pixel-reading process for a still image, information concerning defective pixels having very low levels is described in order to support a high international organization for standardization (ISO) speed rating and a long-time exposure. The data size of the defect-correction data is significantly large.
In contrast, regarding a reading process for a moving image, even when an ISO speed rating that is used is the same as that used in a reading process for a still image, it is not necessary to consider a long-time exposure because the maximum exposure time is defined on the basis of the frame rate. The size of defect-correction data that is generated for a moving image is smaller than that of defect-correction data for a still image. However, even when the defect-correction data for a moving image, the size of which is small, is to be generated, it is necessary to search defective pixels to be corrected in the defect-correction data for a still image, the size of which is significantly larger, and to perform an address conversion. For this reason, the process load is markedly increased.
In a case in which a thinned-out pixel-addition reading process is performed, when not only pixels to be read but also peripheral pixels to be added are defective pixels, the defective pixels influence the address conversion. Accordingly, the performance of the address conversion needs more complicated processing.
Suppose that all types of defect-correction data that can be used in reading methods of the solid-state image-pickup device are to be stored. The size of defect-correction data for a moving image is small. However, it is necessary to prepare defect-correction data for each reading method. More specifically, when a cut-out reading process is supported, it is necessary to prepare defect-correction data for each reading position for each cutting-out ratio. Accordingly, the number of the types of data to be saved is increased, and, in the camera system, the size of the used area of the ROM, in which the defect-correction data is saved, is significantly large. The load necessary for the management is also increased.
For example, suppose that 1/5 of image data is cut out and read in each of the horizontal and vertical directions. Even when reading-start positions and reading-end positions are limited to five positions in the image data in each of both the horizontal and vertical directions, 25 (=5×5) different reading positions can be selected. Because it is necessary to prepare defect-correction data for each of the reading positions, even in this case, the number of types of defect-correction data is 25.
When these data are generated in a manufacturing process of the camera, it is necessary to acquire each of the data for a corresponding reading method. Thus, various factors responsible for the increase in cost, for example, one of which is the increase in test time in the manufacturing process, occur.
In view of the circumstances described above, when pixel data of the solid-state image-pickup device is read in various types of methods to pick up images, the defect-correction process for defective pixels is desirably performed easily at a low cost.